Methods description for DOC and CDOM absorption
Prepared by Antonio Mannino
January 25, 2007
Seawater samples for analysis of dissolved organic carbon (DOC) and chromophoric
dissolved organic matter (CDOM) absorption are filtered under a gentle vacuum (<5 in Hg)
through pre-combusted (6 hours at 450°C) Whatman GF/F glass fiber filters and collected 
directly into pre-cleaned and pre-combusted sample glass bottles and vials.  Duplicate 
samples for DOC analysis are collected and stored frozen.  DOC is measured in triplicate 
(3 of 7 injections to maintain a standard deviation <2%) by high temperature combustion 
oxidation using a Shimadzu TOC-V (Benner and Strom 1993; Sharp et al. 2002).  The deep 
Sargasso seawater reference material (Hansell Laboratory, University of Miami RSMAS) is 
used daily to verify the accuracy of DOC measurements and maintain an analytical error 
to within 5%.  Standard curves of the manufacturer-recommended carbon standard (potassium 
hydrogen phthalate) ranging from 500 to 4000 ug C L-1 are conducted prior to analysis each 
time the catalyst is replaced.  Furthermore, standards are interspersed between every 9 
samples for each sample batch to verify the consistency of the carbon response throughout 
each sample batch analyzed.  The instrument carbon blank is determined from the area counts 
of the numerous Milli-Q ultraviolet oxidized ultra-pure water injections.  The instrument 
carbon blank is subtracted from each sample and deep sea reference standard.  To minimize 
salt accumulation in the TOC-V flow path, Milli-Q water blanks are inserted between every 
3 samples for each batch of samples.  Injection volumes for seawater samples, standards 
and blanks are typically 120uL.  
	
Samples for determination of CDOM spectral absorption coefficients are stored under 
refrigeration.  In the laboratory, CDOM samples are warmed to room temperature and filtered 
through 0.2 um Whatman Nuclepore polycarbonate filters or Gelman Supor (polyethersulfone) 
filters prior to analysis (Mitchell et al. 2000).  Filtration of CDOM samples through GF/F 
filters is necessary for coastal ocean waters due to the high particle load that quickly clog 
Nuclepore and Supor filters.  Absorption spectra of CDOM are measured using a Cary 100 
UV-Visible scanning spectrophotometer (250-800 nm) with UV oxidized Milli-Q water as the 
blank and reference (Mitchell et al. 2003).  The instrument noise for reference baselines of 
air-to-air and ultra-pure water spectral absorption scans is within ±0.0005 absorbance units.  
Spectral absorption coefficients are determined after subtracting the raw absorption 
measurements with field filtration blanks of UV-oxidized Milli-Q and a null point value 
(Mitchell et al. 2000; 2003).  For the work presented here, the absorption spectra of filtration 
blanks are within the level of instrument noise, thus, additional corrections are not necessary.  
The absorption coefficients, ag(Lambda),  are calculated from the following expression:  
a(Lambda)=2.303A(Lambda)/L, where A(Lambda) is the absorbance of filtered seawater measured 
across pathlength L in meters.  Instrument performance tests are conducted each day prior to 
analysis including wavelength accuracy and reproduceability, photometric noise and baseline 
flatness.  NIST-traceable calibration standards (e.g., Holmium oxide filter, etc.) are also 
used to verify instrument performance.  The uncertainty associated with CDOM spectral absorption 
coefficients at an instrument noise level of 0.012 m-1 is on the order of 0.02-0.046 1/m 
(Blough and Del Vecchio 2002).  The CDOM spectral slope coefficient (Sag) is determined by 
fitting a single-exponential non-linear curve [a(Lambda)=a(Lambdao)e-S(Lambda- Lambdao), 
where a(Lambda) and a(Lambdao) represent the absorption coefficients at wavelength Lambda 
and reference wavelength Lambdao], across the 300-700nm scan.   
Benner, R. and M. Strom. 1993. A critical evaluation of the analytical blank associated with DOC 
measurements by high-temperature catalytic oxidation. Mar. Chem. 41: 153-60.
Blough, N.V. and R. Del Vecchio. 2002. Chromophoric DOM in the coastal environment, pp. 509-546. 
In:  Hansell D.A. and Carlson C.A. (eds), Biogeochemistry of Marine Dissolved Organic Matter.  
Academic Press.
Mitchell, B.G., et al. 2000. Determination of spectral absorption coefficients of particles, 
dissolved material and phytoplankton for discrete water samples, pp. 125-153. In: Fargion G.S. 
and Mueller J.L. (eds), Ocean Optics Protocols for Satellite Ocean Color Sensor Validation. 
NASA/TM-2000-209966.
Mitchell, B.G., M. Kahru, J. Wieland and M. Stramska. 2003. Determination of spectral absorption 
coefficients of particles, dissolved material and phytoplankton for discrete water samples, 
pp. 39-64. In: Mueller J.L.,  G.S. Fargion and C.R. McClain (eds), Ocean Optics Protocols for 
Satellite Ocean Color Sensor Validation. NASA/TM-2003-211621/Rev4-Vol.IV.
Sharp, J.H., C.A. Carlson, E.T. Peltzer, D.M. Castle-Ward, K.B. Savidge and K.R. Rinker. 2002. 
Final dissolved organic carbon broad community intercalibration and preliminary use of DOC 
reference materials. Mar. Chem. 77: 239-253.